<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <title>01-font-size</title>
    <style>
        div {
            font-size: 20px;
        }

        /*span {*/
            /*font-size: 200%; !* 200% * 20px = 2 * 20px = 40px*!*/
        /*}*/

        /*span {
            font-size: 30px;
            width: 2em; !* 2 * 30px = 60px *!
        }*/

        span {
            font-size: 2em; /* 2 * 20px = 40px*/
        }
    </style>
</head>
<body>

<div>
    我是div
    <span>我是span</span>
</div>

</body>
</html>